Don't allow content to get mutated when creating glossary-plain (#2274)

* Disallow unlabeled string results

* Don't shift in extractGlossaryStructuredContentRecursive to avoid mutation

Author: Kuuuube

ext/js/templates/anki-template-renderer.js

@@ -787,8 +787,8 @@ export class AnkiTemplateRenderer {
     _extractGlossaryStructuredContentRecursive(content) {
         /** @type {import('structured-content.js').Content[]} */
         const extractedContent = [];
-        while (content.length > 0) {
-            const structuredContent = content.shift();
+        for (let i = 0; i < content.length; i++) {
+            const structuredContent = content[i];
             if (Array.isArray(structuredContent)) {
                 extractedContent.push(...this._extractGlossaryStructuredContentRecursive(structuredContent));
             } else if (typeof structuredContent === 'object' && structuredContent) {
@@ -800,8 +800,6 @@ export class AnkiTemplateRenderer {
                 if (structuredContent.content) {
                     extractedContent.push(...this._extractGlossaryStructuredContentRecursive([structuredContent.content]));
                 }
-            } else if (typeof structuredContent === 'string') {
-                extractedContent.push(structuredContent);
             }
         }